1. Field of the Invention
This invention relates to an ink jet color recording method and, more particularly, to an ink jet color recording method achieving good color recording on any recording paper, e.g., non-coated paper or ordinary paper, such as woodfree paper, non-woodfree paper and bond paper. This invention also relates to an ink set and an apparatus for use with the method.
2. Description of the Related Art
In ink jet recording methods, various ink ejection methods, such as an electrostatic attraction method based on applying a high voltage, a method of causing mechanical vibration or a displacement in a colored ink (ink) by using a piezoelectric element, and a method of utilizing a pressure caused by heating and foaming ink, are used to generate a droplet of ink and to cause the same to fly and attach to a recording member, such as paper, to form an ink dot, thus performing recording. Ink jet recording methods therefore realize low-noise, high-speed and multi-color printing.
For example, recording information or images by such ink jet recording methods may be applied as a surface image for observation using paper or an ink jet recording sheet or the like as a recording member, as a recorded image for observation projected on a screen or the like by using a transparent recording member and an optical apparatus such as an overhead projector, preparation of color-decomposed plates for forming positive plates for color printing, use of a recording member as a color mosaic filter with a color display device such as a liquid crystal device, and an apparel or large-form display using a fabric recording member. Thus, this kind of recording can be applied in various ways and, therefore, attracts attention.
On the other hand, ink jet printers have been developed which are capable of suitably printing monochromatic images on ordinary paper generally used, such as woodfree and non-woodfree paper and ordinary plain paper copier (PPC) paper (electrophotographic recording paper), and which have been realized by an increase in the printing dot density of printing heads and improvements in ink properties. Such ink jet printers are now being widely developed and are being designed as smaller and low-cost units.
However, high-quality images cannot be obtained by color ink jet printers presently available unless coated paper specially developed for ink jet recording is used.
In general, a recording method based on subtractive color mixing can be mentioned as a typical color image forming method based on an ink jet recording method. That is, three primary colors, such as cyan, magenta and yellow, are mixed to express various colors. In this method, ink droplets of the three primary colors, cyan, magenta and yellow, are superposed to express black. In this case, droplets of inks of these colors having approximately equal volumes are used and three ink droplets are attached to a recording member while being superposed on each other. There is, therefore, a problem in that a dot thereby formed spreads excessively in comparison with ink portions where colors other than black are recorded to form a thicker line, resulting in an unnatural image. Also, the amount of ink on the black portion is so large that ink absorption failure of the recording member can occur. This problem is important because the frequency of recording in black is usually high when recording is performed.
To avoid this problem, black is conventionally expressed by using a black ink in addition to inks of the three primary colors, cyan, magenta and yellow.
However, even if recording is performed by using inks of four colors, i.e., cyan, magenta, yellow and black, the problems described below still remain.
(1) If a recording member is ordinary paper such as woodfree paper to which two ink droplets of different colors are jetted in a superposing manner to form a mixed color portion, a certain time is required to absorb the increased amount of ink, and the ink droplets permeate through gaps between paper fibers in all directions as well as in a direction along a cross section of the recording member during the absorption, thereby forming non-uniform dots and causing irregular line thickening (feathering). As a result, an unclear image having reduced edge sharpness is formed.
(2) Even in a case where two ink droplets of different colors are jetted to adjacent two places, the inks are partially mixed at the boundary between the two places before they are fixed, thereby causing boundary bleeding between the different colors and forming an unclear image.
(3) Ink in a color-mixture portion where the amount of jetted ink is increased is not fixed well and it is possible that the recording apparatus or the recording member will be contaminated by contact therebetween or the ink will attach to an operator's hand.
The cause of these problems resides in that the ink previously jetted is not sufficiently fixed in the period of time for jetting the next ink to the adjacent place or to the superposed position.
To solve these problems, methods of using a fast set ink having a high permeation speed such that the fixation time is very short, have been provided. For example, a method such as that disclosed in Japanese Patent Laid-Open Publication No. 55-29546 has been proposed in which the amount of a surfactant in ink is increased in comparison with the conventional ink composition, and ink is made to permeate through paper by utilizing the property of the surfactant improving permeation into cellulose fibers of paper so that the ink apparently dries in an instant.
If such an ink is used, a problem described below is encountered although bleeding can be prevented. That is, by an increase in permeation speed, the penetration of dyestuffs into a recording member is promoted so that the color density is reduced, and wetting on the recording member surface is improved. Therefore, feathering, i.e., a small amount of running of ink along paper fibers, can occur easily around an image of each color. Feathering is unnoticeable in a color image region but noticeable in a black image region and particularly reduces the printing quality. In particular, if the black image region is formed as a black character, the character has an unclear image, low in sharpness and poor in printing quality.
Also in color recording printers, black characters are used most frequently, and the printing quality thereof is important. Therefore, it is preferable, in terms of apparent image impression, to form sharper edges without unnatural edge thickening and to simultaneously set a higher color density in comparison with other colors, when black characters are formed.
To solve these problems and to obtain a high-quality image having reduced feathering and a high density, there is a need to use a black ink having a comparatively low permeation speed. However, if such an ink is used, bleeding of color inks occurs at a boundary portion of a recording image region of each color adjacent to a black region, thereby considerably reducing the printing quality.
On the other hand, Japanese Patent Laid-Open Publication No. 64-63185 discloses a method in which a colorless or light color liquid containing a compound for making a dyestuff in ink insoluble is attached to a portion of a recording member to which the ink is to be thereafter attached. The dyestuff is thereby made insoluble to limit bleeding and feathering and to improve the color-developed density, the resolution and other quality factors.
This method, requiring the operation of previously jetting such a liquid to any printing portion, entails the problems of an increase in total printing time, and an increase in fixation time and cockling of paper due to an increase in the total volume of liquid droplets at each printing portion. Also, this method is disadvantageous in terms of printing cost.